Coaxial variable attenuator switch



y 2, 1953 D. E. ROWE 2,638,503

COAXIAL VARIABLE ATTENUATOR SWITCH Filed Aug. so; 1945 OUTPUT INVENTOR DELMAN E. ROWE "m m A ATTORNEY enough for the probe i I to be inserted completely in the spring-fingered end 9 of the first section 4 of the inner conductor thereof. The rod 6 may also be pulled out of the coaxial line I far enough to establish a substantial gap d between the probe I I and the spring-fingered end 9 of the first inner conductor section 4 with which the probe H cooperates. The width of this aforementioned gap d is therefore variable. mutually directed ends 8 and 9 of the two' sections 4' and l, respectively, of the inner conductor of the coaxial line I are as aforementioned separated by a fixed distance. The rod 8 is adapted electrically and mechanically to vary this aforementioned fixed distance.

Electromagnetic energy may be introduced into or taken from the apparatus at "either end.

In the hereinafter following discussion, it will be;

assumed that such energy is being introduced at the open end of the first coaxial line I and taken from the apparatus at the open end of the second coaxial line 2.

The apparatus of Fig. 1 works in a manner that can be best explained with reference to Fig. 2. Fig. 2 illustrates, a coaxial line I5 which has a center conductor I6 fore-shortened so that one end, here the right-hand end It of the coaxial line I5, is in fact a cylindrical Wave guide. Dotted lines I! illustrate lines of force of the electric field extending from the center conductor I6 to the outer cylindrical conductor I9 of the coaxial line I5. The diameter I8 of the outer cylindrical conductor I9 is of a relatively too. small size for wave guides but of a usual and ordinary size for coaxial lines. Thus the cylindrical wave guide portion I5 is not adapted to carry electromagnetic energy in any wave guide mode at the frequency of operation, as will be hereincelow explained.

In order that the wave guide end I5 of the apparatus of Fig. 2 may function as a wave guide, the diameter I8 thereof must be at least 0.57 times as great as the wave length in free space of energy at the operative frequency. However, the outer conductor I9 of a coaxial line as illustrated in Fig. 2 is usually of much smaller diameter than 0.57 times the wave length of energy at the operative frequency. As a result, the electric field I'I beyond the inwardly terminated end of the inner conductor It will possess a certain amount of energy, and at a frequency below cutoff frequency the impedance to the fiow of this energy in the wave guide section I5 will be exceedingly large, so that the coaxial line may be considered as opened in the electrical sense as well as in the mechanical. Of course, should the frequency of operation increase, this impedance will decrease and displacement current will take Theplace in the hollow cylinder I5 in increasing quantity. In this manner the hollow cylindrical portion I5 would be converted to an operative wave guide. However as aforementioned, the diameter I8 is not sufiiciently great to permit such action, and the hollow cylindrical wave guide section I5 may be said therefore to be a cylindrical wave guide operating at a frequency below cut-off frequency for that wave guide. In the cylindrical wave guide I5 operating below the cut-off frequency, waves are attentuated even through there is no loss of energy. This means that the energy merely fiuctuates between the source namely the coaxial pair I9 and It and the tube or pipe I5. Above the cut-off frequency some energy would pass into the tube I5,

I and travel along that tube. As hereinabove mentioned, the diameter I8 would have to be at least 0.57 times the free space wave length of energy at the operative frequency before waves in even the lowest mode for cylindrical wave guides would be carried by the hollow pipe I5.

Inthe apparatus of Fig. 1, the region in the neighborhood of the end 9 of the first inner conductor section 4 resembles the region in the neighborhood of the inwardly directed end of the inner conductor I6 of the apparatus of Fig. 2. That is, the hollow pipe section of length d is in efiect a cylindrical wave guide when the probe II is pulled back from the thereunto directed end 9 of the first conductor section 4. Lines of electric force similar to the lines I! of Fig. 2 exist in the neighborhood of the therewith cooperating end 9 as the probe II is separated from the said end 9. When the probe I I is within the end 9 of the first inner conductor section 4, positive electrical contact is established for the entire inner conductor comprising sections i and 4 and the rod 6. As the probe II is pulled out of the end 9, the aforementioned lines of electrical force establish electrical contact between the probe II and the end 9. The electrical contact established by the aforementioned lines of force diminishes in strength as the probe II is pulled further and further away from the end 9 of the first inner conductor section 4. Thus there is attenuation of the energy passing through the coaxial line I since the diminishing in electrical contact is tantamount to an increase in impedance to the flow of energy in the coaxial 1ine'I.. When the distance d separating the probe II and the end 9 becomes sumciently great, effective contact between the probe II and the end 9 becomes substantially broken, thereupon causing a switching action which interrupts the flow of energy in the coaxial line I. At this point, as in the apparatus of Fig. 2 a section of the outer conductor I of length d between the probe and the end 9 becomes a cylindrical wave guide which is of too small a diameter to carry energy in any mode for cylindrical wave guides, and therefore, presents a large impedance to signals flowing in the coaxial line I. As aforementioned in connection with the discussion of Fig. 2, the coaxial pair comprising the outer conductor I and the first inner conductor section 4 is practically opened in the electrical sense as Well as in the mechanical sense.

Thus it can be seen that my invention provides a variable attenuator and switch for a coaxial line. Although I have shown and described only a certain specific embodiment of my invention, I am fully aware of the many modifications possible thereof. Therefore my invention is not to be limited except insofar as is necessitated by the prior art and the spirit of the appended claims.

I claim:

1. In a coaxial transmission line an inner conductor comprising first and second hollow cylindrical members coaxially mounted a fixed distance apart and a third cylindrical member telescopically mounted within said first member in slidable electrical contact therewith, means for sliding said third member within said first member in either direction parallel to the longitudinal axis thereof, a tapered electrical probe in the end of said third member directed toward the .mutually directed end of said second member, and means in said last mentioned mutually directed end for establishing slidable electrical 5 contact with said third member when said probe is inserted therein.

2. A variable attenuator and switch for a coaxial transmission line comprising an outer cylindrical conductor of a diameter smaller than that required to carry energy at the operative frequency in any mode for a cylindrical wave guide, an inner conductor having first and second hollow cylindrical members fixedly mounted in said outer conductor a fixed distance apart on the longitudinal axis thereof and a third cylindrical member telescopically mounted within said first member in slidable electrical contact therewith, said third member being of sufficient length to touch both said first and said second members simultaneously and having a tapered electrical probe in its end directed toward said second member, means in said second member in its end facing toward said first member for cooperating with said probe to establish slidable electrical contact therewith when said probe is inserted therein, and means for sliding said third member axially within said first member.

3. A variable attenuator and switch for a coaxial transmission line comprising a first coaxial line having a closedend, a second coaxial line electrically coupled to said first line near said closed end, the inner conductor of said first coaxial line having first and second hollow cylindrical members spaced a fixed distance apart,

said first member being mounted in said closed end and extending therethrough and in electrical contact with the inner conductor of said second coaxial line, a third cylindrical member slidably mounted within said first member in electrical contact therewith and extending outwardly through said closed end and inwardly into said first coaxial line toward said second member, means in the mutually directed ends of said second and third members for establishing slidable electrical contact therebetween, and means comprising the outwardly extending portion of said third member for moving said third member axially with respect to said first coaxial line.

DELMAN E. ROWE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,044,580 Leach June 16, 1936 2,411,553 Ramo Nov. 26, 1946 2,451,201 Clark Oct. 12, 1948 2,514,544 Hansen July 11, 1950 2,527,549 Herring Oct. 31, 1950 FOREIGN PATENTS Number Country Date 17,402 Great Britain of 1890 

